1. Field of the Invention
This invention relates to a novel means for providing improved damping in the control loop of a vibrating sound source used in seismic data acquisition. More particularly, this invention relates to the application of an improved feedback signal developed from the acceleration of the reaction mass in a seismic data acquisition vibrating sound source to the control loop to reduce vibratory seismic signal applied to the earth's surface.
2. Background Art
In seismic exploration, a data acquisition system is employed wherein a sound source is applied to the surface of the earth and sound pickup devices known as geophones are attached to the surface of the earth to pick up resulting echoes from subsurface strata. Recordings of the signals received by these geophones are made and the relative positions of the echoes on the respective time axes of the recordings are used to locate the depths of these strata. Information thereby gained is used to locate valuable minerals, principally oil and gas, for subsequent drilling operations.
A principally used sound source is the explosion of dynamite charges. Selection of the force of the explosion and its location relative to the location of the geophone array allow a controlled sound impulse to be directed into the earth so that the resulting echoes will provide the strata location as desired. The danger of injury and property damage from the explosions, as well as the resulting noise pollution, has restricted the use of explosions as a sound source to remote areas. In order to provide a sound source without these deleterious effects, hydraulically driven vibratory sound sources have been developed. The vibratory sound source is usually implemented with a hydraulically-actuated shaker plate mounted under a heavy truck. Hydraulic force is transmitted to the shaker plate by a shaft from a hydraulic piston and cylinder assembly mounted in a heavy reaction mass structure. The shaker plate is lowered to the surface of the earth, and a vibratory signal is thus directed into the earth for subsequent reception by geophones. The vibratory sound source may be used in populated areas without danger of serious injury, property damage or excessive noise pollution.
The hydraulic cylinder and piston assembly generally employs two cylinder chambers, one above the piston and one below the piston. The top and bottom piston faces, generally of substantially equal area, are exposed to pressure from hydraulic fluid introduced into the two chambers. By selecting the chamber which has the greater pressure, the piston may be moved up or down as desired. The magnitude of the net force on the piston is proportional to the pressure differential.
The vibratory sound source replaces the sound impulse of the explosive charge with a swept frequency vibratory signal of several seconds duration. It may be shown that the resulting geophone signals can be conditioned by a filter having a linear frequency vs. delay characteristic so as to produce signals like those resulting from explosive sources. The waveshape of the transmitted vibratory signal applied to the surface of the earth must be precisely controlled so as to reduce distortion of the received signals and allow correct matching of the conditioning filter to the transmitted signal. Adequate control of the large mass shaker plate and associated hydraulic driving mechanism requires properly tailored feedback loops in the shaker plate drive control system.
Much work in the art of vibratory sound has been done in developing relevant feedback signals, conditioning the feedback signals and applying the conditioned feedback signals to the shaker plate drive mechanism so as to improve performance. One such effort is that taught in U.S. Pat. No. 3,216,525 issued to Fail et al. A technique known as Force Control, as exemplified in U.S. Pat. No. 4,184,144, uses a feedback signal proportional to the force actually applied to the surface of the earth by the shaker plate developed from other measurable parameters in the system.
One problem encountered in shaker plate drive control systems is the presence of low frequency resonance peaks in the drive control transfer function. These peaks typically occur in the region of 5 to 50 Hertz. The resonance peaks result from a combination of the natural modes of resonance of the shaker plate and its associated large mass drive system, and resonant effects caused by rock strata located near the point on the earth's surface where the shaker plate is applied. Excessive resonance can cause the shaker plate to decouple, that is, rise slightly from the earth's surface when vibrating. This results in distortion of the sound signal actually imparted into the earth.